Plasma-enhanced chemical vapor deposition (PECVD) is a chemical-based process used to deposit thin semiconductor material films or layers on a substrate such as a wafer in the formation of integrated circuit devices such as chips and dies. PECVD machines or tools generally include a reaction or process chamber configured for holding the wafer. A reactant or process gas containing the desired film material chemical precursors is introduced into the chamber and energized by a power source that generates an RF (radio frequency) (AC) or DC signal sufficient to excite a capacitive discharge and form an ionized gas plasma above the wafer. The reaction grows or deposits a generally conformal thin film on the wafer surface which can include a wide array of film materials such as silicon dioxide (SO2), silicon oxynitride (SiON), polysilicon, silicon nitride (SiN), dielectrics, etc.
The thicknesses of films formed during PECVD are sometimes not uniform and vary across the surface of the wafer. This can be attributed to factors such as the process chamber design, RF electrode arrangement, and gas injection system configuration and pressures, etc. The resultant RF plasma field produced sometimes preferentially favors the central portion of the wafer where the plasma field is often strongest and chemical reaction deposition rates therefore highest. Accordingly, the semiconductor film formed near the edge or peripheral regions of the wafer may be thinner than the central region of the wafer. Non-uniformity in film thickness may compromise subsequent wafer fabrication steps, and adversely affect the integrity and performance of IC devices built on the wafer thereby increasing die reject rates. Moreover, difficulty in maintaining film uniformity across the wafer can be expected to become more problematic as wafer sizes increase from the 300 mm to larger 450 mm sizes that allow more IC devices to be built on a single substrate and offer associated economies.
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